Radioactivity
Measurements

Radioactivity is a
measure of a radionuclides activity, i.e., by the
number of atoms that disintegrate per unit time.
The conventional unit for activity is the curie (Ci).
A curie is defined by the activity in one gram of
naturally occurring Radium-226 and equals 37-billion
disintegrations per second. The Systeme
International d'Unites (SI) is the recognized
international standard for describing measurable
quantities and their units. The standard SI unit
for activity is the becquerel (Bq). A becquerel is
equal to one disintegration per second.

Radiation Exposure and Dose

The primary concern of radioactivity is the amount
of energy deposited by particles or gamma radiation
to the surrounding environment. Of particular
concern is the amount of damage this energy can
cause living tissue. When radiation interacts with a
substance, it interacts with electrons of atoms
which can alter the number of electrons associated
with those atoms (ionization).

The
term “exposure” is used to express the amount of
ionization produced in air by electromagnetic (gamma
and X-ray) radiation. The unit of exposure is
roentgen (R). The average exposure rate from natural
radioactivity in southeast Idaho is about 0.130 R
per year. Exposure applies only to electromagnetic
radiation in air. Absorbed dose describes the amount
of energy from ionizing radiation absorbed by any
kind of matter. When absorbed dose is adjusted to
account for the amount of biological damage a
particular type of radiation causes, it is known as
dose equivalent. The unit for dose equivalent is
called the rem (“roentgen-equivalent-man”). The SI
unit for dose equivalent is called the seivert (Sv).
One seivert is equivalent to 100 rem.

Sources of Radiation

Unit Prefixes

The range of numbers experienced in many scientific
fields, like that of environmental monitoring for
radioactivity, is huge and units for very small and
very large numbers are commonly expressed by
scientists as a prefix that modifies the unit of
measure. One example is the prefix kilo, abbreviated
k, which means 1,000 of a given unit. A kilometer is
therefore equal to 1,000 meters. Prefixes used in
ESER reports include:

Prefix

Abbreviation

Meaning

Mega

M

1,000,000 (= 1 x 106)

milli

m

0.001 (= 1 x 10-3)

micro

µ

0.000001 (1 x 10-6)

pico

p

0.000000000001 (= 1 x 10-12)

Scientific Notation

Scientific notation is used to express numbers which
are very small or very large. A very small number
will be expressed with a negative exponent, e.g.,
1.2 x 10-6. To convert this number to the more
commonly used form, the decimal point must be moved
left by the number of places equal to the exponent
(in this case, six). Thus the number 1.2 x 10-6 is
equal to 0.0000012. A large number will be expressed
with a positive exponent, e.g. 1.2 x 106. To convert
this number, the decimal point must be moved right
by the number of places equal to the exponent. For
example, number 1.2 x 106 is equal to 1,200,000.

Concentrations of Radioactivity

The amount of radioactivity in a substance of
interest is described by its concentration which is
described as the amount of radioactivity per unit
volume or weight of that substance. Air, milk, and
atmospheric moisture samples are expressed as
activity per milliliter (mL). Concentrations in
surface and drinking water and precipitation samples
are expressed as activity per liter (L).
Radioactivity in foodstuff and soil are expressed as
activity per gram (g). Exposure, as measured by
environmental dosimeters, is expressed in units of
milliRoentgens (mR). This is sometimes expressed in
terms of dose as millirem (mrem) or microseiverts (µ
Sv